about sloped sides ofuro

Hi, Great looking products! I live on Maui in Hawaii and have
had a hinoki O-Furo since 2004. It is located outside, open air, but
covered with a roof and about a 100 meters from the ocean. We have used
it every day, 5 months of the year and keep water in it, with occasional
draining and refills. We use an electronic water purifier with
occasional small doses of chlorine (which I want to stop) and the wood
has stayed in pretty good shape, although it does feather occasionally.
The outside bottom has just started to rot and will require replacement
in the near future. The current tub is 5 feet in diameter, 27 inches
deep, and has straight sides. My first question is: Is there any reason
for sloped sides other than aesthetics? I prefer the look of sloped
sides, but I am not sure it will be easy to retrofit.
Thank you for your interest in our japanese bathtubs!
We made barrel type ofuro (maru-buro) with almost straight edges even if some slant is needed to cling on the metal bands.
If you like it slanted, no problem: this is the normal shape.